1. Field of the Invention
This invention relates to toughening the matrix portion of molten alkali metal carbonates electrolyte during impregnation with active electrolyte and to providing high matrix surface area under fuel cell operating conditions resulting in stable long term fuel cell operation. The toughening is achieved by both crack deflection and resistance to fiber pullout by inclusion of electrically non-conducting, electrolyte compatible fibers within the matrix.
2. Description of Related Art
Various attempts have been made to reinforce the matrix portion of molten alkali metal carbonates electrolytes to obtain higher electrical output over longer periods of time. U.S. Pat. No. 4,079,171 teaches reinforcement of lithium aluminate electrolyte matrices with about 2 to about 6 volume percent, preferably about 2 to about 4 volume percent, of a metallic alloy containing principally iron, chromium and about 2 to about 6 weight percent aluminum, in the form of fine screens, woven mesh, expanded sheets, fibers or particles, specifically disclosing use of Kanthal fibers 0.005 inch in diameter and 0.5 inch long. U.S. Pat. No. 4,216,278 contains similar disclosure, specifically disclosing use of Kanthal wire mesh (20.times.20 wires inch.sup.-1, 0.005 inch diameter wire) pressed into a lithium aluminate electrolyte matrix.
U.S. Pat. Nos. 4,538,348 and 4,581,302 disclose molten carbonate fuel cell matrices having about 5 to about 30 volume percent of corrosion resistant ceramic particulate material functioning primarily for crack attenuation. These particulate materials have an average size range greater than about 25 microns, preferably greater than about 50 microns, of the same material as the less than 1 micron sized inert particulates making up the major portion of the matrix, or of a more reactive material, such as alumina due to less reactivity of the large particle size.
The mechanical properties of silicon carbide whisker/aluminum oxide matrix composites having 30 volume percent silicon carbide whiskers having an average diameter in the sub-micron range and an average length in the order of about 10 to 30 microns is described in Vaughn, Wallace L., Homeny, Joseph, and Ferber, Mattison K., "Mechanical Properties of Silicon Carbide Whisker/Aluminum Oxide Matrix Composites" Ceram. Eng. Sci. Proc., 8 (7-8), pp. 848-859, (1987).